1、Organic ChemistryHaloalkanesStructure a compound containing a halogen covalently bonded to an sp3 hybridized carbon;given the symbol RX a compound containing a halogen bonded to an sp2 hybridized carbon a compound containing a halogen bonded to a benzene ring;given the symbol ArX(we do not study vin
2、ylic or aryl halides in this chapter)Nomenclature number the parent chain to give the substituent encountered first the lowest number,whether it is halogen or an alkyl group indicate halogen substituents by the prefixes fluoro-,chloro-,bromo-,and iodo-,and list them in alphabetical order with other
3、substituents locate each halogen on the parent chain by giving it a number preceding the name of the halogen in haloalkenes,number the parent chain to give carbon atoms of the double bond the lower set of numbersNomenclature examples name the alkyl group followed by the name of the halideB Br r2 2-B
4、 Br ro om m o o-4 4-m m e et th hy yl l-p pe en nt t a an ne e123454-Bromo-cyclohexene123456trans-2-Chloro-cyclohexanolBrC Cl lO O H HB Br rC Cl lC Cl l2 2-B Br ro om m o ob bu ut ta an ne e(s se ec c-B Bu ut ty yl l b br ro om m i id de eC C n nh hl lo or ro oe et th he en ne e(V V i in ny yl l c c
5、h hl lo or ri i d de e)3 3-C C h hl lo or ro op pr ro op pe en ne e(A A l ll ly yl l c ch hl lo or ri id de e)Nomenclature several polyhaloalkanes are common solvents and are generally referred to by their common or trivial names hydrocarbons in which all hydrogens are replaced by halogens are commo
6、nly named as perhaloalkanes or perhaloalkenesC CH H C Cl l3 3C CH H2 2C Cl l2 2C CC Cl l2 2=C CH HC Cl lC CH H3 3C CC Cl l3 3 D D i ic ch hl lo or ro om m e et th ha an ne e(M M e et th hy yl le en ne e c ch hl lo or ri id de e)T Tr ri ic ch hl lo or ro om m e et th ha an ne e (C C h hl lo or ro of
7、fo or rm m)T Tr ri ic ch hl lo or ro oe et th hy yn ne e (T Tr ri ic ch hl lo or r)1 1,1 1,1 1-T Tr ri ic ch hl lo or ro oe et th ha an ne e (M M e et th hy yl l c ch hl lo or ro of fo or rm m)P Pe er rc ch hl lo or ro oe et th hy yl le en ne eP Pe er rf fl lu uo or ro op pr ro op pa an ne eP Pe er
8、rc ch hl lo or ro oe et th ha an ne eC CC CC Cl lC Cl lC Cl lC Cl lC Cl lC Cl lF F C CC CC CF FF FF FF FF FF FF FC Cl lC CC CC Cl lC Cl lC Cl lDipole MomentsuDipole moment of RX depends on:the sizes of the partial charges the distance between them the polarizability of the unshared electrons on halo
9、genC CH H3 3C Cl lC CH H3 3F FC CH H3 3B Br rC CH H3 3I IH H a al lo om m e et th ha an ne eE El le ec ct tr ro on ne eg ga at ti iv vi it ty yo of f H H a al l o og ge en nC C a ar rb bo on n-H H a al lo og ge en nB Bo on nd d L Le en ng gt t h h (p pm m)D D i ip po ol le e M M o om m e en nt t(d d
10、e eb by ye es s;D D)4 4.0 03 3.0 02 2.8 82 2.5 51 13 39 91 17 78 81 19 93 32 21 14 41 1.8 85 51 1.8 87 71 1.8 81 11 1.6 62 2van der Waals ForcesuHaloalkanes are associated in the liquid state by van der Waals forces a group intermolecular attractive forces including dipole-dipole forces dipole-induc
11、ed dipole forces induced dipole-induced dipole(dispersion)forcesuvan der Waals forces pull molecules together as molecules are brought closer and closer,van der Waals attractive forces are overcome by repulsive forces between electron clouds of adjacent atoms or moleculesvan der Waals Forces the ene
12、rgy minimum is where the attractive forces are the strongest nonbonded interatomic and intermolecular distances at these minima can be measured by x-ray crystallography and each atom and group of atoms can be assigned a nonbonded atoms in a molecule cannot approach each other closer than the sum of
13、their van der Waals radii without causing nonbonded interaction strain I In nc cr re ea as si in ng g v va an n d de er r W W a aa al ls s r ra ad di iu us sH HF FC Cl lB Br rC CH H2 2C CH H3 3I I1 12 20 01 13 35 51 18 80 01 19 95 52 20 00 02 20 00 02 21 15 5Boiling PointsuFor an alkane and a haloal
14、kane of comparable size and shape,the haloalkane has the higher boiling point the difference is due almost entirely to the greater polarizability of the three unshared pairs of electrons on halogen compared with the low polarizability of shared electron pairs of covalent bonds a measure of the ease
15、of distortion of the distribution of electron density about an atom in response to interaction with other molecules and ions;fluorine has a very low polarizability,iodine has a very high polarizabilityC CH H3 3C CH H3 3C CH H3 3B Br rb bp p -8 89 9 C Cb bp p 4 4 C CBoiling Points among constitutiona
16、l isomers,branched isomers have a more compact shape,decreased area of contact,decreased van der Waals attractive forces between neighbors,and lower boiling points2 2-B Br ro om m o o-2 2-m m e et th hy yl lb bu ut ta an ne eb bp p 7 72 2 C CB Br r1 1-B Br ro om m o ob bu ut ta an ne eb bp p 1 10 00
17、 0 C CB Br rBoiling Points boiling points of fluoroalkanes are comparable to those of hydrocarbons of similar molecular weight and shape the low boiling points of fluoroalkanes are the result of the small size of fluorine,the tightness with which its electrons are held,and their particularly low pol
18、arizabilityH H e ex xa an ne e(M M W W 8 86 6.2 2,b bp p 6 69 9 C C)F F1 1-F Fl lu uo or ro op pe en nt ta an ne e(M M W W 9 90 0.1 1,b bp p 6 63 3 C C)C CH H3 3C CH HC CH H3 3C CH H3 3C CH H3 3C CH HC CH H3 3F F2 2-F Fl lu uo or ro op pr ro op pa an ne eM M W W 6 62 2.1 1,b bp p -1 11 1 C C2 2-M M
19、e et th hy yl lp pr ro op pa an ne eM M W W 5 58 8.1 1,b bp p -1 1 C CDensityuThe densities of liquid haloalkanes are greater than those of hydrocarbons of comparable molecular weight a halogen has a greater mass per volume than a methyl or methylene groupuAll liquid bromoalkanes and iodoalkanes are
20、 more dense than wateruDi-and polyhalogenated alkanes are more dense than waterC CH H X X3 3C CH H2 2X X2 2C CX X4 4C Cl lB Br rI IX X=1 1.4 48 83 32 2.4 49 97 74 4.0 00 08 8D D e en ns si it t y y (g g/m m L L)a at t 2 25 5 C C3 3.3 32 25 52 2.8 89 90 01 1.5 59 94 43 3.2 27 73 34 4.2 23 3H H a al l
21、o oa al lk ka an ne e1 1.3 32 27 7Bond Lengths,StrengthsuC-F bonds are stronger than C-H bonds;C-Cl,C-Br,and C-I bonds are weakerC C-H HC C-F FC C-C Cl lC C-B Br rC C-I IB Bo on nd dB Bo on nd dL Le en ng gt th h(p pm m)B Bo on nd d D D i is ss so oc ci ia at ti io on n E Et th ha al lp py y k kJ J
22、(k kc ca al l)/m m o ol l 1 10 09 91 14 42 21 17 78 81 19 93 32 21 14 44 41 14 4 (9 99 9)4 46 64 4 (1 11 11 1)3 35 55 5 (8 85 5)3 30 09 9 (7 78 8)2 22 28 8 (5 57 7)Halogenation of AlkanesuIf a mixture of methane and chlorine is kept in the dark at room temperature,no change occursuIf the mixture is
23、heated or exposed to visible or ultraviolet light,reaction begins at once with the evolution of heat a reaction in which an atom or group of atoms is replaced by another atom or group of atomsC CH H4 4C Cl l2 2C CH H3 3C Cl lH HC Cl l+h he ea at t+M M e et th ha an ne eC Ch hl lo or ro om m e et th
24、ha an ne e(M M e et th hy yl l c ch hl l o or ri id de e)Halogenation of Alkanes if chloromethane is allowed to react with more chlorine,a mixture of chloromethanes is formedC CH H3 3C Cl lC Cl l2 2C CH H2 2C Cl l2 2H HC Cl l+h he ea at t+C C h hl lo or ro om m e et th ha an ne e(M M e et th hy yl l
25、 c ch hl lo or ri i d de e)D D i ic ch hl lo or ro om m e et th ha an ne e(M M e et th hy yl le en ne e c ch hl lo or ri id de e)C CH H2 2C Cl l2 2C Cl l2 2C CH HC Cl l3 3C Cl l2 2C CC Cl l4 4h he ea at th he ea at tD D i ic ch hl lo or ro om m e et th ha an ne e(M M e et th hy yl le en ne e c ch hl
26、 lo or ri id de e)T Tr ri ic ch hl lo or ro om m e et th ha an ne e(C C h hl lo or ro of fo or rm m)T Te et tr ra ac ch hl lo or ro om m e et th ha an ne e(C Ca ar rb bo on n t te et tr ra ac ch hl lo or ri id de e)RegioselectivityuRegioselectivity is high for bromination,but not as high for chlorin
27、ationC CH H3 3C CH H2 2C CH H3 3B Br r2 2C Cl l2 2C CH H3 3C CH H2 2C CH H3 3C CH H3 3C CH H C CH H3 3C Cl lC CH H3 3C CH H C CH H3 3B Br rC CH H3 3C CH H2 2C CH H2 2C Cl lC CH H3 3C CH H2 2C CH H2 2B Br rH H B Br rH H C Cl lP Pr ro op pa an ne e+h he ea at to or r l li ig gh ht t+2 2-B Br ro om m o
28、 op pr ro op pa an ne e(9 92 2%)1 1-B Br ro om m o op pr ro op pa an ne e(8 8%)P Pr ro op pa an ne e+h he ea at to or r l li ig gh ht t+2 2-C Ch hl lo or ro op pr ro op pa an ne e(5 57 7%)1 1-C C h hl lo or ro op pr ro op pa an ne e(4 43 3%)RegioselectivityuRegioselectivity is 3 2 1 for bromination,
29、approximately 1600:80:1 for chlorination,approximately 5:4:1 draw all monobromination products and predict the percentage of each for this reaction+h he ea at t2 2-M M e et th hy yl lp pr ro op pa an ne eC C H H3 3C C H H3 3C C H H3 3C C H HB B r r2 2C C4 4H H9 9B Br r+H H B B r rEnergeticsuBond Dis
30、sociation Enthalpies(BDE)C CH H2 2=C CH H-H HC CH H2 2=C CH H C CH H3 3-H HC CH H3 3 C CH H3 3C CH H2 2-H HC CH H3 3C CH H2 2(C CH H3 3)2 2C CH H-H H(C CH H3 3)2 2C CH H(C CH H3 3)3 3C C-H H(C CH H3 3)3 3C C C C6 6H H5 5C CH H2 2-H HC C6 6H H5 5C CH H2 2 C CH H2 2=C CH HC CH H2 2-H HC CH H2 2=C CH H
31、 C CH H2 2 B Be en nz zy yl lA A l ll ly yl lV V i in ny yl lM M e et th hy yl lE Et th hy yl lI Is so op pr ro op py yl lt te er rt t-B Bu ut ty yl l3 37 72 2 (8 89 9)3 37 76 6 (9 90 0)4 40 05 5 (9 97 7)4 41 14 4 (9 99 9)4 42 21 1 (1 10 01 1)4 43 39 9 (1 10 05 5)4 46 64 4 (1 11 11 1)H H y yd dr ro
32、oc ca ar rb bo on nR R a ad di ic ca al lN N a am m e e o of fR R a ad di ic ca al l EnergeticsuUsing BDE,we can calculate the heat of reaction,H0,for the halogenation of an alkaneC CH H4 4C Cl l2 2C CH H3 3C Cl lH HC Cl l+4 43 39 9(+1 10 05 5)+Mechanism any chemical species that contains one or mor
33、e unpaired electrons radicals are formed by the order of stability of alkyl radicals is 3 2 1 methylC CH H3 3C CH H2 2O OO O C CH H2 2C CH H3 3C Cl lC Cl lC CH H3 3C CH H3 3C Cl l C CH H3 3 C CH H3 3C CH H2 2O O C Cl l C CH H3 3 O O C CH H2 2C CH H3 3 H H0 0 =+1 15 50 0 k kJ J/m m o ol l (+3 36 6 k
34、kc ca al l/m m o ol l)E Et th ho ox xy y r ra ad di ic ca al ls sD D i ie et th hy yl l p pe er ro ox xi id de e8 80 0+D DH H0 0 =+3 37 77 7 k kJ J/m m o ol l (+9 90 0 k kc ca al l/m m o ol l)M M e et th hy yl l r ra ad di ic ca al ls sE Et th ha an ne eh he ea at t+C Ch hl lo or ri in ne e a at to
35、om m s sC Ch hl lo or ri in ne el l i ig gh ht t+D DH H0 0=+1 15 50 0 k kJ J/m m o ol l (+3 36 6 k kc ca al l/m m o ol l)Radical Chain Mechanism a step in a chain reaction characterized by formation of reactive intermediates(radicals,anions,or cations)from nonradical or noncharged moleculesl l i ig
36、gh ht t+C Cl lC Cl lC Cl l C Cl lo or r h he ea at tS St te ep p 1 1:Radical Chain Mechanism a step in a chain reaction characterized by the reaction of a reactive intermediate and a molecule to form a new reactive intermediate and a new molecule the number of times the cycle of chain propagation st
37、eps repeats in a chain reactionC CH H3 3C CH H2 2 C CH H3 3C CH H2 2H HC CH H3 3C CH H2 2 C CH H3 3C CH H2 2C Cl lH HC Cl l C Cl l C Cl lC Cl lC Cl l+S St te ep p 2 2:S St te ep p 3 3:+Radical Chain Mechanism a step in a chain reaction that involves destruction of reactive intermediatesC CH H3 3C CH
38、 H2 2C CH H3 3C CH H2 2C Cl lC CH H2 2C CH H3 3C Cl lC Cl lC CH H3 3C CH H2 2C CH H2 2-C CH H2 2H HC Cl lC Cl lC CH H3 3C CH H2 2-C Cl lC CH H3 3C CH H3 3C CH H3 3C CH H2 2-C CH H2 2C CH H3 3C CH H2 2=C CH H2 2 S St te ep p 6 6:S St te ep p 7 7:+S St te ep p 4 4:S St te ep p 5 5:+Chain Propagation S
39、tepsuFor any set of chain propagation steps,their equations add to the observed stoichiometry enthalpies add to the observed H0 C CH H3 3C CH H2 2-H H C Cl lC CH H3 3C CH H2 2 C Cl l-C Cl lC CH H3 3C CH H2 2-H HC Cl l-C Cl lC CH H3 3C CH H2 2 C CH H3 3C CH H2 2-C Cl lC CH H3 3C CH H2 2-C Cl l C Cl l
40、H H-C Cl lH H-C Cl l+4 42 22 2 (+1 10 01 1)-4 43 31 1(-1 10 03 3)+2 24 47 7(+5 59 9-3 35 55 5(-8 80 0)-9 9 (-2 2)-1 10 08 8 (-2 26 6)+-1 11 17 7 (-2 28 8)Regioselectivity?uThe regioselectivity of chlorination and bromination can be accounted for in terms of the relative stabilities of alkyl radicals
41、(3 2 1 methyl)uBut how do we account for the greater regioselectivity of bromination(1600:80:1)compared with chlorination(5:4:1)Hammonds Postulate the structure of the transition state for an exothermic step looks more like the reactants of that step than the products for an endothermic step looks m
42、ore like the products of that step than the reactantsuThis postulate applies equally well to the transition state for a one-step reaction and to each transition state in a multi-step reactionHammonds Postulate in halogenation of an alkane,hydrogen abstraction(the rate-determining step)is exothermic
43、for chlorination but endothermic for brominationH HH H-B Br rH H-B Br rC Cl lC Cl lB Br rB Br rH H-C Cl lH H-C Cl lH H-4 43 31 1 (-1 10 03 3)+54(+13)-431(-103)+405(97)+-368(-88)-26(-6)+37(+9)+Reaction step-9(-2)-368(-88)+422(101)+422(101)+405(97)17(4)17(4)HH Hammonds PostulateuBecause hydrogen abstr
44、action for chlorination is exothermic:the transition state resembles the alkane and a chlorine atom there is little radical character on carbon in the transition state regioselectivity is only slightly influenced by radical stabilityHammonds PostulateuBecause hydrogen abstraction for bromination is
45、endothermic:the transition state resembles an alkyl radical and HBr there is significant radical character on carbon in the transition state regioselectivity is greatly influenced by radical stability radical stability is 3 2 1 methyl,and regioselectivity is in the same orderStereochemistryuWhen rad
46、ical halogenation produces a chiral center or takes place at a hydrogen on a chiral center,the product is a mixture of R and S enantiomers as a racemic mixture for simple alkyl radicals,the carbon bearing the radical is sp2 hybridized and the unpaired electron occupies the unhybridized 2p orbital(se
47、e next screen)C CH H3 3C CH H2 2C CH H2 2C CH H3 3B Br r2 2C CH H3 3C CH H2 2C CH HC CH H3 3B Br rH HB Br r+B Bu ut ta an ne e(R R,S S)-2 2-B Br ro om m o ob bu ut ta an ne eh he ea at to or r l l i ig gh ht t+StereochemistryAllylic Halogenation a C adjacent to a C-C double bond an H on an allylic c
48、arbon an allylic C-H bond is weaker than a vinylic C-H bond+P Pr ro op pe en ne e3 3-C C h hl lo or ro op pr ro op pe en ne e (A Al ll ly yl l c ch hl lo or ri id de e)3 35 50 0 C CC C H H2 2=C C H H C C H H3 3C C l l2 2C C H H2 2=C C H H C C H H2 2C C l lH H C C l lH HC CC CC CH HH HH HH HH H+4 46
49、64 4 k kJ J (1 11 11 1 k kc ca al l)/m m o ol l+3 37 72 2 k kJ J (8 89 9 k kc ca al l)/m m o ol lAllylic BrominationuAllylic bromination using NBSN NB Br rO OO Oh h +N-Bromo-succinimide(NBS)+Allylic BrominationuA radical chain mechanism Chain initiation Chain propagationNOOBrh +C C H H2 2=C C H H C
50、C H H2 2-H HB B r rC C H H2 2=C C H H C C H H2 2H H-B Br rC C H H2 2=C C H H C C H H2 2B B r r-B Br rC C H H2 2=C C H H C C H H2 2-B B r rB B r rAllylic Bromination chain terminationuBr2 is provided by the reaction of NBS with HBr B B r r B B r rC C H H2 2=C C H H C C H H2 2 C C H H2 2=C C H H C C H
